Many different systems have been proposed for setting the cruise control in a motor vehicle, for example, a motor car. The most common approach is to use a signal from vehicle speed sensor, which may be the same as the speedometer sensor, as a reference signal which is compared with a second reference signal which is held at a steady value following activation of the cruise control by the driver. The speed signal will, in general, vary as the vehicle is being driven, for example, owing to changes in road gradient, head wind, or the demands placed upon the engine power output from accessories such as a vehicle air conditioning unit.
The difference between the speed and reference signals may then be used as measure of whether or not the engine is supplying the right power to maintain the vehicle speed with the set cruise control speed. Usually the difference is used to physically control an engine throttle setting, for example, through an actuator linked to the accelerator pedal. One example of a prior art system is disclosed in EP 0 353 072 Al, in which a throttle value is physically controlled after a dedicated engine control unit has made in software a simultaneous selection of a maximum of an idle, cruise or driver demand signals.
Apart from the need to design a dedicated engine control unit, certain disadvantages have been noted with this general approach. First, the need to control the throttle physically with an actuator adds cost and complexity, compared with a vehicle having no such actuator. A limitation of some systems which rely upon an actuator is that the cruise control speed is not set immediately upon activation of the cruise control by the driver but takes some seconds to become activated as the actuator moves from an off position to a position corresponding to the current accelerator pedal setting.
Another difficulty arises from providing space for the physical components associated with an actuator, and its supply of power, within the limited space available in the engine compartment. The provision of these components adds cost, and consequently cruise control is often an optional accessory.
Another disadvantage in vehicles having electronic engine management (EEM) systems is that the relationship between pedal position and throttle setting is dependent upon many parameters, such as engine temperature, and in the case of an automatic transmission other parameters may include the gear selected, the rate of change of pedal position, and changes of gear. Inclusion of the cruise control feedback loop within the driver demand part of the EEM system increases the complexity of designing and programming the engine management system.
A further problem is that the vehicle speed sensor, for example, a sensor which detects a rotating magnet or which uses the Hall effect, may not give a precise indication of vehicle speed and may provide a speed signal which lags the actual vehicle speed. The cruise control system may therefore be slow in responding to changes in vehicle speed.